1. Field of the Invention
The present invention generally relates to a temperature detecting system and method, and is particularly concerned with such a system used for detecting human presence in an area, either for security purposes or for control of lighting and HVAC (Heating, Ventilation and Air Conditioning) so that such features operate only when humans are present.
2. Related Art
In order to reduce energy consumption in homes and other buildings, automatic systems are often used to control lighting and HVAC (“Heating, Ventilation and Air Conditioning”) so that such comfort and convenience features operate only when humans are present. Automatic systems employ sensors to determine human presence. Commonly, several types of motion sensors are used, including Passive Infrared (PIR) motion sensors and active Doppler motion sensors (either microwave- or ultrasound-based). The commonly used sensors share a common trait: they are designed to detect human motion.
In most situations (particularly indoor situations where the climate is controlled by HVAC systems) humans are warmer than the objects that surround them (typically, walls, floors and ceilings). As infrared (IR) radiation produced by an object depends on temperature, humans typically emit more infrared (IR) radiation than do objects that surround them. This IR radiation can be monitored by detectors that are sensitive in the 8-14 micron IR wavelength range. One type of IR detector, often called a pyroelectric infrared detector, functions by means of the piezoelectric effect. Changes in IR radiation incident on its detecting element cause deformation of the element, and subsequent measurable charge transfer between two “plates” on the element. This type of detector is highly sensitive, yet only to changes in radiation intensity—not to absolute radiation intensity. Due to such high sensitivity, pyrolectric detectors are widely used in motion sensors. Motion sensors employ one or more sets of optics that direct radiation to an IR detector from a field-of-view. When a human enters and leaves the field-of-view, the detector reports, respectively, changes in IR radiation intensity, first from that of the less radiant background to that of the more radiant human, and then back to that of the background. This sequence can be interpreted by the sensor as having arisen from human motion. Since pyroelectric detectors do not respond to static radiation intensity, such sensors have no means to detect the continued presence of a human that has stopped moving.
As noted above, motion sensors are not completely sufficient for detecting human presence. For example, in a residence, a person may sit in a chair and read a book for several hours without moving enough to be detected by a motion sensor. In a business, a meeting might last for several hours without any of the participants moving significantly. In such cases, an automatic system can create a minor nuisance by extinguishing room lighting or de-activating HVAC while persons are present.
Other detectors, such as thermocouple arrays, can measure static IR radiation intensity. Such detectors can be used to detect the static presence of a “warmer” object in a room (such as a human). However, they have certain disadvantages relative to pyroelectric detectors, including relatively high cost. Thus, they are not used in commercial human presence sensors.
Therefore, what is needed is a system and method that reduces or overcomes these significant problems found in the conventional systems as described above.